1. Field of the Invention
The present invention relates to an anamorphic converter disposed at an image side of an image-forming optical system to convert an aspect ratio of an image. In addition, the present invention also relates to a lens system and a shooting system including the anamorphic converter and used in a television camera, a video camera, etc.
2. Description of the Related Art
Various devices for recording/reproducing images with converted aspect ratios have been suggested. In particular, in a typical picture recording/reproducing system of the cinemascope-format with an aspect ratio of 2.35:1 used for motion pictures, an image is optically reduced in the horizontal direction with an anamorphic lens before the image is recorded on film, and then the image on the film is optically enlarged in the horizontal direction with the anamorphic lens when the image is projected onto a screen. In addition, various front converters disposed at an object side of image-forming optical systems have been suggested for use as anamorphic converters. Examples of front converters are disclosed in Japanese Patent Publication No. 48-24048 (see line 34 in the left column to line 32 in the right column of page 2 and FIG. 1), Japanese Patent Laid-Open No. 2-13916 (see lines 10 to 16 in the lower right column of page 2 and FIG. 1), Japanese Patent Laid-Open No. 3-25407 (see line 14 in the upper left column to line 6 in the upper right column of page 4 and FIG. 1), Japanese Patent Laid-Open No. 5-188271 (see paragraphs 0023 to 0025 and FIG. 1), Japanese Patent Laid-Open No. 5-188272 (see paragraphs 0022 to 0023 and FIG. 1), Japanese Patent Laid-Open No. 6-82691 (see paragraphs 0013 to 0015 and FIGS. 1(A) and 1(B)), and Japanese Patent No. 2817074 (see paragraph 0012 and FIGS. 1 to 3).
In addition, rear converters disposed at the image side of image-forming optical systems have also been suggested. An example of a rear converter is disclosed in Japanese Patent No. 3021985 (see paragraphs 0016 to 0017 and FIGS. 1 to 3).
Recently, image quality has improved in the field of video technology and digital cinema systems in which motion pictures shot with high-definition television (HDTV) systems have become popular. In the digital cinema systems, imaging devices with an aspect ratio of 16:9 (1.78:1) are generally used. Accordingly, there are demands for anamorphic converters with which pixels of the imaging devices can be effectively used and the image quality can be improved so that the digital cinema systems can be used for shooting motion pictures with the cinemascope-format with the aspect ratio of 2.35:1.
In addition, there are other various formats for motion pictures such as American vista with an aspect ratio of 1.85:1 and European vista with an aspect ratio of 1.66:1.
In anamorphic converters for digital cinemas, it is preferable that desired aspect-ratio conversion be performed and shading of light from an object not to occur. In addition, preferably, an effective image plane of the image-forming optical system is efficiently used, reduction in the amount of peripheral light is small, and high optical performance is obtained at the entire zoom/focus area. It is also important that the above-described various aspect ratios are obtainable.
As described in Japanese Patent Laid-Open Nos. 2-13916 and 6-82691 and Japanese Patent No. 2817074, front converters are advantageous in that they have simple structures and shading can be prevented irrespective of the conversion magnification by setting a suitable effective diameter. However, astigmatism varies during focusing. Although systems for correcting the astigmatism during focusing are suggested in Japanese Patent Publication No. 48-24048 and Japanese Patent Laid-Open Nos. 3-25407, 5-188271, and 5-188272, a correcting unit included in the converter must be driven in association with the focusing performed by the image-forming optical system, and a complex mechanism is necessary.
In addition, since the size is increased to obtain a sufficient field angle and an F-number, a plurality of large converters having different magnifications must be prepared for obtaining different aspect ratios.
In comparison, rear converters are advantageous in that astigmatism does not vary during focusing.